Mobile networks are currently evolving from pure circuit switched (CS) networks towards IP based networks. In doing so, they integrate and are and by that integrated into IP based infrastructures that are is also used for the Internet, the World Wide Web and the datacom industry in general.
With this trend mobile networks closely follow the evolution steps of the wireline networks, where VoIP via DSL access or via WLAN access is existing technology today.
Mobile operators of mobile communication networks, such as GSM/WCDMA operators, that install IMS (IP Multimedia Subsystem) networks and offer IMS services want to make these IMS services available also to GSM/WCDMA subscribers. Hereto, calls from and to this group of (GSM/WCDMA) subscribers need to be routed through the IMS network in order to reach a respective IMS service engine offering the respective service. This concept is called also known as IMS Centralized Services (ICS). The IMS centralized Centralized Services work item in 3rd Generation Partnership Program (3GPP) targets at using the Circuit Switched access for access to IMS services, see 3GPP Technical Specification TS 23.292, and is complemented by IMS Service Continuity, see 3GPP TS 23.237.
In parallel to the ICS trend the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS), supporting Evolved UTRAN (eUTRAN or E-UTRAN) (or LTE Long Term Evolution, as it is also called) as new radio access network. As part of this discussion, work on single radio voice call continuity (SRVCC) is ongoing in 3GPP SA2 (see 3GPP TS 23.216), within said framework would allow for enabling to transfer an IMS voice call from EPS to CS. It is envisioned that SR-VCC is combined with ICS for an IMS centralized solution supporting also SRVCC from UTRAN/GERAN CS to E-UTRAN.
In the FIG. 1A displaying an exemplary architecture figure, an MSC Server which is enhanced for SRVCC has either an ISUP (ISDN User Part) or an SIP (Session Initiate Protocol) interface.
Within said architecture a transfer of an IMS voice call from EPS/LTE towards GERAN/UTRAN CS is conceptually shown. In the lower part, the User Equipment UE being connected via the E-UTRAN towards the IMS network via the Serving and PDN gateway is shown. This User Equipment is now to be transferred (as indicated by the arrow on the left side) towards the UTRAN/GERAN CS in which it would be connected to the IMS network via the Target UTRAN/GERAN and the MSC-Server. A HSS (Home Subscriber Server) is only shown for completeness but are not necessary for the understanding of the invention.
FIG. 1B displaying an exemplary flowchart, illustrating a handover from an LTE/EPS system (E-UTRAN) towards a UTRAN/GERAN CS system as exemplified in the architecture of FIG. 1A.
Within said scheme due to some reasons, e.g. measurement reports as indicated in step 1 of FIG. 1B lead to decision in the source, i.e. in the E-UTRAN (lower portion of FIG. 1B) to initiate a handover towards a target, i.e. UTRAN/GERAN CS in the upper portion of FIG. 1B. The relocation request is sent towards the MSC-Server which than prepares the Target network (Target MSC, Target SGSN, Target RAN (UTRAN/GERAN)) for the handover (steps 4.-.7). Thereafter in a step 8, the session transfer towards the IMS is initiated. After step 8, the Service Centralization and Continuity Application Server (SCC AS) in the IMS (an SCC AS correlates Service Control Signaling and CS Bearer Control Signaling and presents session to IMS) has performed the session transfer for speech media, i.e., the remote end within the IMS has been updated to send speech media to the Media Gateway (MGW) controlled by the MSC Server enhanced for SRVCC and not longer over E-UTRAN to the User Equipment (UE).
Similar procedure allows a SRVCC from UTRAN PS to UTRAN/GERAN CS. In this case RNC would correlate to eNodeB and SGSN correlate to MME above.
While the eNode B as part of the Source UTRAN has decided in step 2 to perform a handover (decision to HO) (which causes the Mobility Management Entity or the Mobility Management Node (MME or SGSN) to send a Forward Relocation Request towards the MSC Server), it is allowed that the eNode B part of the Source UTRAN may decide later that it is not longer required to perform handover, e.g., due to newer measurement reports. In such a case and in case of a Packet Switched (PS) handover, the eNodeB would inform the MME accordingly (or the NodeB would inform the SGSN) and the PS handover procedure would be stopped and all bearers would remain in EPS.
However, when the eNode B would decide to not require handover after the session transfer has been initiated by the MSC Server enhanced for SRVCC (step 9), the UE would stay in E-UTRAN but the speech media would already be send to the target MSC Server enhanced for SRVCC. Thereby the user would not be able to continue his phone call, since the media and the related SIP control signaling would be directed towards the target MSC Server/MGW and not via E-UTRAN.
The process described above pertains to proposals for a session transfer from a CS access towards a LTE access for IMS calls that had been initiated via LTE access, particularly handed over to a CS access and then handed over back to LTE access. However, this has been considered as too limited. So far, no solution is known which would allow for a handover of CS to PS accessed calls, wherein the call has been originated in the CS domain or call has been handed over to CS domain. A solution is needed which works also for calls being initiated via CS access.